The present invention relates to a lead frame for a semiconductor device, and a method of producing the same.
A lead frame having a die-pad for mounting a semiconductor element thereon and inner leads for leading out electrodes of the semiconductor element has been widely used in a semiconductor device. In a semiconductor element mounted on such a lead frame die-pad, as shown in FIG. 1, electrodes 22 of a semiconductor element are respectively connected to corresponding inner leads 3 through wires 23. Recently, however, the number of the electrodes 22 of the semiconductor element 21 is so large that the pitch of the electrodes 22 is extremely narrow (generally, about 100-180 .mu.m).
On the other hand, the inner leads 3 are formed of a lead frame material such as so-called 42-alloy through etching or punching, that is, using a punch press and referred to hereinafter as "press working". As shown in FIG. 2, generally, the width w of each of the inner leads 3 is selected to be within a range of from about 80 to 150 .mu.m, the thickness t of the same is selected to be within a range of from about 100 to 300 .mu.m, the pitch p of the inner leads is selected to be within a range of from about 150 to 250 .mu.m, and the gap g between the inner leads is selected to be within a range of from about 70 to 170 .mu.m.
In producing the inner leads 3, portions of the lead frame material corresponding to the gaps (g in FIG. 2) between the inner leads 3 are removed through etching or punched off through press working. In the case of etching, it takes much time because the thickness t of the lead frame material is so thick, and therefore the opposite side surfaces of each of the inner leads 3 are removed in a large arc as shown in FIG. 3, so that each of the inner leads 3 is formed in the shape like a hand drum to make the sectional area small. As a result, it is difficult to assure the mechanical strength of each inner lead 3 in the direction of its thickness, so that the inner lead 3 is often crushed or deformed when the wire 23 is bonded to the inner lead 3.
Further, in the case where the inner leads 3 are formed through press working there are defects as follows. That is, since the thickness t of each inner lead 3 is larger than the width w of the same, each inner lead 3 is twisted so that the gap between adjacent inner leads 3 is made wider (or narrower in a not-shown case) at their lower end sides as shown in FIG. 4 when the lead frame material is punched from its top by means of a downward press, while each inner lead 3 is twisted so that the gap between adjacent inner leads 3 is made narrower (or wider in a not-shown case) at their upper end sides as shown in FIG. 5 when the lead frame material is punched from its bottom by means of an upward press. Accordingly, sometimes, any of the wires 23 cannot come into perfect contact at it whole surface with the corresponding inner lead 3, resulting in defective connection.